1. Field of the Invention
This invention relates generally to a bidirectional signal transmission network and more particularly to a bidirectional signal transfer shift register which is suitably incorporated, for example, in a drive circuit of an active matrix liquid crystal display device and used for reversal display of an image.
2. Description of the Related Art
An active matrix liquid crystal display device is conventionally known and has such a a general construction as shown in FIG. 6. Referring to FIG. 6, the active matrix liquid crystal display device (LCD) 100 shown includes a liquid crystal layer 101 which exhibits a predetermined electro-optical effect and is held between a pair of transparent glass substrates 102 and 103. A plurality of picture element electrodes 104 are formed on an inner surface of the substrate 102 and disposed in a matrix. A thin film transistor 105 made of polycrystalline silicon or a like material is connected to each of the individual picture element electrodes 104. The drain electrodes of the thin film transistors 105 are connected to the corresponding picture element electrodes 104; the source electrodes are connected to corresponding data lines 106; and the gate electrodes are connected to corresponding gate lines 107. A plurality of opposing electrodes 108 are formed on an entire inner surface of the other substrate 103. An image data signal is written by way of the individual thin film transistors 105 into the corresponding picture element electrodes 104. Electro-optical variations of the liquid crystal layer 101 caused by the image data signal are detected as transmission light amount variations by means of, for example, a pair of polarization plates (not shown) to effect display of a desired image.
An active matrix liquid crystal display device having the construction described above can be utilized, for example, as a light valve for a liquid crystal projector. A liquid crystal projector includes three liquid crystal display devices to which the three primary colors are allocated, and a common enlarging projection lens system. The liquid crystal display devices function as light valves for the different color systems of red, green and blue. The liquid crystal display devices resolve a primary image into red, green and blue components and display them. Red, green and blue illumination light beams are introduced simultaneously into the liquid crystal display devices. Single color transmission light images of the individual liquid crystal display devices are composed by means of a dichroic prism or a dichroic mirror, and a thus composed full color image is projected in an enlarged scale onto a screen by the projection lens system. In the optical system of the liquid crystal projector, a primary image is composed after reflection reversal is repeated several times. Depending upon the arrangement structure of the optical system, the number of times of reflection reversal is different among the different color systems. Accordingly, in order to obtain an aligned full color image, a primary image component of a predetermined color must be displayed reversely in advance. Or, depending upon installation environments of the liquid crystal projector, it may possibly be mounted in an inverted posture on a ceiling to effect projection. Also in this instance, a primary image to be displayed on the liquid crystal display device must be reversed in advance.
In this manner, a structure which can suitably select reversal display depending upon an object of use or an application of the liquid crystal display device is conventionally demanded. To this end, various image reversing systems have been proposed conventionally. For example, such a system as shown in FIG. 7 which makes use of image signal processing is known. Referring to FIG. 7, an original image data signal SIG is inputted once to an analog to digital (A/D) conversion and reversal processing circuit 110, in which it is converted into a digital signal and then processed by reversal processing. For example, the data signal is successively written into a frame memory and then read out in the reverse direction. The data signal processed by the reversal processing is supplied to a liquid crystal display device 100 by way of a buffer 111.
FIG. 8 shows another system for image reversal. Referring to FIG. 8, a downward scanning circuit 120 is connected to gate lines 107 of a liquid crystal display device 100. An ordinary unidirectional shift register not shown is incorporated in the downward scanning circuit 120 and sends out a gate signal to the gate lines 107 successively beginning with the upper end and ending with the lower end of the screen. Also an upward scanning circuit 121 is connected to other gate lines 107. A unidirectional shift register not shown is incorporated also in the upward scanning circuit 121 and sends out a gate signal to the gate lines 107 successively beginning with the lower end to the upper end of the screen. The downward scanning circuit 120 and the upward scanning circuit 121 in pair can be selected suitably. When the downward scanning circuit 120 is selected, a normal or non-reversed image is displayed, but when the upward scanning circuit 121 is selected, a vertically reversed image is displayed. Similarly, a rightward scanning circuit 122 and a leftward scanning circuit 123 in pair are connected to data lines 106. When the rightward scanning circuit 122 is selected, normal image display is performed, but when the leftward scanning circuit 123 is selected, a leftwardly and rightwardly reversed image is displayed. It is to be noted that, while the four scanning circuits are shown disposed outside the liquid crystal display device 100 in FIG. 8, it is possible to actually form them in an integrated condition in the inside of the liquid crystal display device 100.
In the reversing system shown in FIG. 7 which makes use of image signal processing, since the A/D reversal processing circuit and so forth have a large scale, there is a problem to be solved in that the power dissipation is high and reduction in size is obstructed, resulting in disadvantage in terms of the cost. Meanwhile, with the reversing circuit structure shown in FIG. 8, there is another subject to be solved in that, since four scanning circuits are required, where they are formed in the inside of a liquid crystal display device, the device requires a large area and the yield is deteriorated as much.